Noncortical origins of the spinal motor evoked potential in rats.

Motor evoked potentials (MEPs) were recorded from the spinal cord, sciatic nerve, or both during transcortical electrical stimulation in the rat. Four peaks could be consistently identified in the spinal MEP. The latency and amplitude of the peaks varied differentially with intensity and polarity of stimulation. Conduction velocity for Peak 1 of the MEP was 43 m/sec. Bilateral sciatic nerve MEPs were present after unilateral cortical stimulation. The spinal MEP was elicited by stimulation of areas outside the motor cortex, and the response persisted during subcortical stimulation and after motor cortex ablation. We present evidence suggesting that components of the spinal MEP in rats arise from pathways outside the motor cortex.